1. Field of the Invention
The present invention relates to a method and apparatus for generating a train of carrier suppressed optical pulses, and a grating device.
2. Description of the Related Art
In recent years, communication traffic demands are rapidly increasing due to a proliferation of the Internet, and high speed and large capacity transmission systems at transmission rates of 1 terabits per second (1 Tbit/s=1×1012 bits/s) or higher are becoming commercially practical. To implement the transmission rate of the order of terabits, a wavelength division multiplexing (WDM) technique is indispensable. Particularly, attention is directed to a dense WDM (DWDM) system which densely arranges wavelengths of optical carriers (channels) with narrower wavelength intervals. However, the DWDM system implies a problem in a deterioration of transmission characteristics due to nonlinear effects such as overlapping (cross-talk) of spectra, inter-phase modulation, and the like between adjacent channels.
Noted as a method for solving this problem is a transmission system which employs a carrier suppressed return-to-zero (CS-RZ) signal that has a narrower spectrum bandwidth than a conventional RZ (Return-to-Zero) signal and exhibits a high tolerance to the nonlinear effects. The CS-RZ signal is generated, for example, by a method which uses a Mach-Zender (MZ) phase modulator, for example, described in a paper entitled “320 Gbit/s (8×40 Gbit/s) WDM Transmission over 367-km Zero-Dispersion-Flattened Line with 120-km Repeater Spacing Using Carrier Suppressed Return-to-Zero Pulse Format” by Y. Miyamoto et al. (OAA '99, PDP4, 1999). Alternatively, the CS-RZ signal is generated by a method which uses a phase shifter and a delay line, for example, described in a paper entitled “Generation of Multi-Wavelength Carrier Suppressed RZ Signal with Supercontinuum Light Source” by Konishi et al. (IEICE Technical Report, Vol. 101, No. 394, OCS2001-69, 2001). A method using a mode-locked laser is also described in a paper entitled “Mode-Locked Lasers for 43 Gb/s Carrier Suppressed Return-to-Zero Pulse Generation” by K. Sato et al. (ECOC'2001, 2001).